Alkali hydrides in org. synthesis

[espen180]

I am interested in reactions of the type

C₂H₅COCH₃ + KH -> C₂H₅COCH₂K + H₂

In this paticular reaction, I understand that the participating hydrogen is beta to the carbonyl group since the oxygen's electronegativity makes the beta-hydrogen atoms acidic, and KH acts like a base. However, would the reaction take place had the reagent been an alkane, for example C₄H₁₀? If so, at what carbon atom will the reaction most likely take place?

 

[chemisttree]

Nope. pKa for hydride is something like 28-29 whereas that of an alkane is around 50.

Sodium hydride is sold as a solid coated with a hydrocarbon oil for safety purposes.

 

[DDTea]

espen: watch your terminology. We only describe carbons as alpha, beta, gamma, etc. with respect to a carbonyl. As such, those are alpha hydrogens, NOT beta hydrogens.

Also, to expand on your understanding a bit more: the reason those alpha hydrogen are more acidic than the corresponding alkane is not only due to inductive electron withdrawal by the carbonyl group. If the alpha carbon were deprotonated, there would be resonance stabilization of this anion (you can draw this out for yourself to see). This is important in the haloform reaction.

One of the best ways that I've found to approach questions of reactivity in organic chemistry is to imagine a bond breaking, then asking, "What features of this molecule would stabilize this resulting structure?" This gets to the heart of some chemical principles.

 

[espen180]

Thanks for the help!